A wireless local area network (WLAN) client device may store in its memory one or more profiles of wireless local area networks. A profile may include, for example, the service set identity (SSID) of the network, WLAN configuration parameters, security credentials, and Internet Protocol (IP) network parameters. The SSID is also known as the network name. The profiles may be assigned priorities, for example, by a user that has purchased the client device, a carrier who controls the sale of the client device, or an administrator of an enterprise that has purchased the client device.
Scanning is the process of identifying existing networks. Upon being operatively coupled to a radio of the client device, a WLAN controller of the client device may automatically initiate passive scanning. In passive scanning, the WLAN controller generally listens for beacon frames broadcast by access points (APs), one communication channel at a time. The communication channels, and the time spent passively scanning on a communication channel, are defined by the WLAN standard and/or regulatory requirements. Beacon frames of a WLAN are broadcast at regular intervals, for example, roughly each 100 ms. The SSIDs of the profiles stored in the client device are compared to the SSIDs included in beacon frames received by the client device on a particular communication channel. The order in which SSIDs of the profiles are compared to the SSIDs of the scan results may be determined by one or more factors. For example, the SSIDs of the profiles may be compared in order of decreasing priority for the profiles. In another example, the SSIDs of the scan results may be compared in order of decreasing received signal strength. If the SSID field of a received beacon frame matches the SSID of a particular profile, the WLAN controller may initiate an authentication process and, if the authentication process is successful, may initiate an association or re-association process with the AP that sent the beacon frame. If the comparison does not result in any matches, then the client device may present a list of identified networks (based on the SSID fields of any received beacon frames) to the user of the client device, so that the user can select which, if any, of the networks to join.
In active scanning, the WLAN controller transmits an active probe request including an SSID on a communication channel on which the regulatory requirements allow active probing. The WLAN controller may receive one or more probe responses. The probe responses may be received within 15 ms of the transmission of the active probe request. The active probe request may include a particular SSID, in which case probe responses will be received from APs in the vicinity that are hosting a WLAN having the particular SSID. Alternatively, the active probe request may include a “wild card” for an SSID and probe responses may be received from more than one network. In the latter case, the SSIDs of the profiles stored in the client device are compared to the SSIDs included in the probe responses. The order in which SSIDs of the profiles are compared to the SSIDs of the probe responses may be determined by one or more factors. For example, the SSIDs of the profiles may be compared in order of decreasing priority for the profiles. In another example, the SSIDs of the probe responses may be compared in order of decreasing received signal strength. If the SSID included in a probe response matches the SSID of a particular profile, the WLAN controller may initiate an authentication process and, if the authentication process is successful, may initiate an association or re-association process with the AP that sent the probe response. If no probe responses are received, the client device may transmit the active probe request on a different communication channel, or may transmit an active probe request including a different SSID, or may put its radio into a low-power state.
A WLAN may have a suppressed SSID, in which case beacon frames for that WLAN will not include the suppressed SSID. Likewise, an AP having a suppressed SSID may ignore probe requests including a “wild card” SSID, or may respond with a probe response which lacks the SSID. If the client device stores one or more profiles having suppressed SSIDs, the client device may actively scan for those profiles using an active probe request including the suppressed SSID. An AP having a suppressed SSID will respond to such an active probe request with a probe response which includes the suppressed SSID, if the SSID of the network handled by the AP matches that in the active probe request.
For battery-operated client devices, the process of network discovery and association in a timely manner consumes significant battery life.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.